It is known from U.S. Pat. No. 2,726,215 to use alkali metal salts of azelaic or sebacic acids as corrosion inhibitors for aqueous solutions. British patent specification No. 995 708 describes the use of alkanolamine salts of azelaic and sebacic acids in aqueous drilling and cutting fluids based on polyoxyalkylenes. However, the corrosion-protective properties of salts of azelaic and sebacic acids are not optimal and leave room for improvement, as can be seen from Tables 1 and 3.
It is the state of the art, per the disclosure of U.S. Pat. No. 2,442,672, to add alkylsuccinic acid to hydrocarbon oils as a rust inhibitor, while U.S. Pat. No. 2,971,915 describes the addition of alkyladipic acids having a short side chain, such as methyladipic acid. Alkylsuccinic acids have the disadvantage in aqueous media of forming insoluble alkaline earth metal salts with the hardness-causing compounds of water. These thus precipitate out. On the other hand, short-chain alkyladipic acids, even in the form of their amine salts, still provide inadequate rust protection (cf. Table 1).
It is further known from British patent specification No. 984 409 and U.S. Pat. No. 3,087,936 to add amine salts of substituted succinic acids to lubricating oils for purification purposes and/or for sludge prevention. These amine salts have substituents having a C number of .gtoreq.30. On the one hand, these references do not teach how to prevent metal corrosion in aqueous systems; on the other hand, the respective amine salts involved do not possess sufficient water solubility for such purposes anyway due to the high C number of their substituents.
Under practical conditions, for economic reasons, it is still customary to utilize aqueous fluids containing alkanolamines and sodium nitrite for simple metal machining, for example for grinding and polishing. However, it has been found that carcinogenic nitrosamines are thus formed from nitrite and alkanolamines. In addition, two other commercial products are utilized in a large number of cases as rust-inhibiting additives for metal- machining agents in aqueous systems. Commercial product A is the triethanolamine salt of an acrylsulfonamidoalkylenecarboxylic acid (German Patent No. 1,298,672); commercial product B is a mixture of fatty acid diethanolamide and fatty acid diethanolamine salts. Both commercial products, as seen from Table 1, achieve a rust-protective effect which, especially in the low concentration range, still needs improvement.